1. Field of the Invention
The present invention generally relates to elevators and, in particular, relates to a system for position loss recovery for an elevator car.
2. Description of the Prior Art
In many elevator applications, the absolute position, i.e. location, of the elevator car is determined by use of a machine mounted primary position transducer. Generally, in one typical arrangement, the primary position transducer generates pulses in relation to the distance moved by the elevator car within the hoistway. The pulses are counted relative to an initialization hoistway switch which is typically located at one end of the hoistway, for example, the bottom of the hoistway. Hence, at any time during operation, the location of the elevator can be accurately determined by performing what is generally referred to as a terminal position recovery run. In a terminal position recovery run, the elevator is moved to one end of the hoistway whereat the initialization switch is actuated. The position of the elevator car is thereafter known from counting the pulses produced by the primary position transducer. Such an operation compensates for elevator car position errors, such as rope creep and rope slippage.
When power is lost during operation, however, a position recovery difficulty arises because an elevator car can be required to recover its position only to the nearest landing, or floor, for example, in Fireman's Service Operation, before resuming normal operation. Consequently, a terminal position recovery run is unacceptable.
One basic approach to ensure compliance with such requirements is to maintain power to the necessary circuits and transducers during the powered down condition. Typically, these approaches require that the loss of power be detected and the instantaneous position of the elevator car be identified and stored in a non-volatile memory. Hence, when power is restored, the elevator system controller can access the non-volatile memory to precisely ascertain the current location of the elevator car.
Such approaches, in order to function with the various AC/DC drive systems, require the provision of a secondary power supply, for example, in the form of a battery or by stored capacitive energy. These components are not only expensive and bulky but, in the case of batteries, require maintenance and routine replacement. Further, capacitive energy storage devices are not only bulky but of questionable reliability.
Numerous position location sensors are known in the art. In some elevator systems, e.g., hydraulic systems, magnet assemblies are positioned in the hoistway at various points near the landings to indicate to the approaching car its positions relative to the landing, for leveling, direction, etc. In geared systems, locating vanes are mounted on a tape stretched from top to bottom of the hoistway for similar purposes. The vanes are of differing configurations to signify the differing conditions such as up/down stopping vanes, door zone vanes, floor zone vanes, etc. Both the above mentioned hydraulic and geared position indicating systems use position indicating devices in the hoistway but do not generally distinguish one floor from another. For example, U.S. Pat. No. 4,362,224, entitled Discrete Position Location Sensor, issued on Dec. 7, 1982 to Fairbrother and assigned to the assignee hereof and U.S. Pat. No. 4,346,788, entitled Tailorable Discrete Optical Sensor, issued on Aug. 31, 1982 to Shung and assigned to the assignee hereof describe discrete position sensors that include light sources and optical detectors that are interruptible by a vane. Typically, the vanes are adjustable so that the relative position of two objects can be determined.
In U.S. Pat. No. 4,375,057, entitled Position Sensor, issued on Feb. 22, 1983 to Weiss et at. and assigned to the assignee hereof, microwave signals are used to determine the position of an elevator car in a hoistway. As discussed therein, a slotted microwave waveguide is disposed within the hoistway and a waveguide shorting stub is connected to the elevator car. The waveguide shorting stub slides within the waveguide as the elevator car moves within the hoistway. Hence, by injecting microwave signals into the waveguide and measuring the reflected signal, the position of the waveguide shorting stub, and thus the elevator car, can be readily and accurately determined.
In U.S. Pat. No. 4,363,026 entitled Position Encoder Update Mechanism and Method, issued on Dec. 7, 1982 to Salmon and assigned to the assignee hereof and U.S. Pat. No. 4,384,275 entitled High Resolution And Wide Range Shaft Position Transducer Systems, issued on May 17, 1983 to Masel et al. also assigned to the assignee hereof, the transducers for determining the position of an object include a gear arrangement.
U.S. Pat. No. 4,627,518 entitled Backup Position Signaling In An Elevator, issued on Dec. 9, 1986 to Meguerdichian et at. and assigned to the assignee hereof describes an elevator car position transducer that includes a backup power supply. U.S. Pat. No. 5,274,203 also shows a battery backup along with several fail-safe features including redundancy, misindication checks, etc. It also discloses an absolute position encoder for instant floor position recovery. This last system is very effective but also expensive.
Although each of these techniques has advantages for specific applications, none provide an inexpensive, highly accurate solution to the problem of recovering the position, or location, of an elevator car to the nearest floor subsequent to a power loss. Consequently, a system for position loss recovery for an elevator car that avoids the above recited drawbacks is clearly desirable.